Discovery of an 11-s X-Ray Pulsar in the Galactic-Plane Section of the Scorpius Constellation

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Abstract
During a galactic-plane survey with ASCA in 1996 September, we detected a relatively bright, soft source at R.A. = 17h8m46s.6, DEC. = −40°9′27″ (J2000), and discovered an 11-s X-ray pulsation from the source. This source has been identified with the ROSAT source 1RXS J170849.0−400910. From a timing analysis of the source, we obtained a barycentric pulse period of P = 10.99759±0.00005 s with a broad sinusoidal shape of a pulse fraction of ∼ 30%. The energy spectrum in the 0.8–10 keV region is very soft, and can be fitted by a power-law model with a photon index of 3.5 and an absorption column density of 1.8 × 1022 cm−2. The observed pulse-phase-averaged flux in the range 0.8–10 keV is 4.3 × 10−11 erg cm−2 s−1, which corresponds to 1.7 × 10−10 erg cm−2 s−1 after correcting for soft X-ray absorption. During an observation interval of about 14 hr, neither a significant change in the pulsation period, nor a significant variation in the phase-averaged flux was detected. From these X-ray properties, we suggest that this newly discovered X-ray pulsar might be a member of a small subgroup of “anomalous” X-ray pulsars with a period close to 6–9 s.

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